A. Field of the Invention
The present invention relates generally to infaltable packers for use in oil and gas wells for providing annular seals between the outside of pipe and the surrounding surface of the borehole or casing, and more particularly to a method an inflatable packer and of preparing an inflatable packer having a long inflatable sleeve for insertion into a fluid filled well bore. The method insures that the inflatable sleeve does not creep or expand with respect to the mandrel due to frictional or other forces during running.
B. Description of the Prior Art
Inflatable packers of the type disclosed, for example, in U.S. Pat. No. 3,640,723, or U.S. Pat. No. 3,837,947, have been used for many years. Such packers include a tubular mandrel that is covered by an inflatable sleeve secured to the mandrel by a pair of axially spaced apart end assemblied. Each end assembly includes a collar, which is adapted to be connected to the mandrel, and an annular head, which is connected to one end to the collar, and at the other end to the sleeve. The sleeve is normally reinforced by a reinforcing sheath, which comprises a plurality of overlapping ribs connected at each end to a head. The heads are spaced radially apart from the mandrel, thereby to form an annular cavity radially inside the heads. A passage with valve means is provided in one of the collars for allowing the passage of fluid from inside the pipe string to one of the annular cavities and thence between the inflatable sleeve and the mandrel to inflate the sleeve into sealing contact with the well bore or casing. Such inflatable packers function to isolate the annulus above the packer from that below, and, accordingly, need be only of a length long enough to form an effective seal.
More recently, there have been developed inflatable packers for use in well completion, which are adapted to be positioed adjacent the producing zone and inflated with cement. After the cement has set, the packer is perforated and the well is produced through the packer. Examples of such inflatable packers are disclosed, for example, in U.S. Pat. No. 3,918,522, U.S. Pat. No. Re. 30,711, and U.S. Pat. No. 3,909,034. Such inflatable packers tend to be relatively long, i.e. from ten feet to forty feet in length, in order to seal against both the producing formation, which is perforated, and the formations above and below the producing formation.
Since the completion type inflation packers are of such length the central portion of the inflatable sleeve is supported and, in effect, reinforced by the borehole. Accordingly, a reinforcing sheath is unnecessary in the central part of the inflatable sleeve. However, reinforcing is necessary adjacent the ends of the inflatable sleeve to prevent the inflatable sleeve from blowing out and/or extruding past the heads. Therefore, the inflatable sleeves of the completion type packers are normally reinforced only at the ends adjacent to the heads.
Inflatable packers are intended for use in high pressure environments. Since the well bores into which teh packers are run are filled with drilling fluid, or the like, during running and prior to inflation, the packers may be subjected to extremem hydrostatic pressures. Additionally, during inflation, the inflating fluid or cement is injected into the packer at a pressure substantially higher than the local hydrostatic pressure. Accordingly, inflatable packers must be constructed and prepared so as to be able to withstand both hydrostatic and inflation pressures. The primary failure point due to hydrostatic pressures is at the heads. It will be recalled that the heads are spaced radially apart from the mandrel to define a cavity. If the cavity is maintained at a pressure substantially equal to atmospheric, then the extreme hydrostatic pressures in the well bore can apply collapsing forces to the heads. The primary failure points due to inflation pressures are at the connections between the collars and heads, and heads and sleeves, and bursting of the sleeve itself.
In order to prepare inflatable packers for high pressure service, such packers heretofore have been tested hydrostatically at pressures on the order of the differentials experienced during inflation. During such hydrostatic testing, the packers have been filled with a test fluid, which is usually water, at an appropriate pressure. After hydrostatic testing, a portion, but not all, of the test fluid is drained out. After such draining, the portion of the packer exterior of the mandrel, including the inflatable sleeve and heads, remains filled with fluid. The hydrostatic forces within the well bore act on the retained fluid and substantially equalize the pressure within the cavity or chamber within the heads, thus preventing collapse of the heads.
The above described hydrostatic testing and fluid filling procedure has been effective inpreventing or reducing the number of occurrences of pressure related failures. However, such procedure has contributed to or exacerbated another failure point in inflatable packers. As inflatable packers are run into the well bore, there is sometimes contact between the inflatable sleeve and the borehole wall. Such contact is particularly likely in deviated holes. Contact between the sleeve and the borehole wall during movement causes frictional forces to be applied to the sleeve. Such contact also causes frictional forces to be applied between the interior of the sleeve and the mandrel. If the frictional forces between the exterior of the sleeve and the borehole wall are greater than those between the interior of the sleeve and the mandrel, the sleeve will tend to move with respect to the mandrel. It is well known in general that the magnitude of the frictional force between two surfaces may be expressed by the following equation: EQU F.sub.f =.mu.F.sub.n
where
F.sub.f is the frictional force; PA1 .mu. is the coefficient of friction between the surfaces; and PA1 F.sub.n is the normal force between the surfaces. PA1 F.sub.fsw is the frictional force between the sleeve and well bore; PA1 .mu..sub.sw is the coefficient of friction between the sleeve and the well bore; and PA1 F.sub.nsw is the normal force between the sleeve and the well bore. PA1 F.sub.fsm is the frictional force between the sleeve and the mandrel; PA1 .mu..sub.sm is the coefficient of friction between the sleeve and the mandrel; PA1 F.sub.nsm is the normal force between the sleeve and the mandrel due to contact of the sleeve with the well bore; and PA1 F.sub.H is the force due to any hydrostatic pressure differential between the outside of the sleeve and the inside of the sleeve.
In the case of an inflatable packer being inserted into a well bore, the frictional force between the sleeve and the well bore is expressed as follows: EQU F.sub.fsw =.mu..sub.sw F.sub.nsw
where
The frictional force between the interior surface of the sleeve and the mandrel may be expressed by the following equation: EQU F.sub.fsm =.mu..sub.sm (F.sub.nsm +F.sub.h)
where
The equation expressing the frictional force between the sleeve and mandrel may be rewritten as follows: EQU F.sub.fsm =.mu..sub.sm F.sub.nsm +.mu..sub.sm F.sub.h
The frictional force between the sleeve and mandrel thus has two terms: .mu..sub.sm F.sub.Nsm and .mu..sub.sm F.sub.H. If the pressure within the sleeve is balanced to be equal to that outside the sleeve, as when there is an incompressible fluid such as water between the sleeve and mandrel, the second term is zero. Thus, when hydrostatic forces are eliminated or balanced, the frictional force between the sleeve and the mandrel may be expressed as follows: EQU F=.sub.fsm =.mu..sub.sm F.sub.Nsm
It will be noted that with respect to both the frictional forces between the sleeve and wall bore and the mandrel and the sleeve, the normal forces, F.sub.NSW and F.sub.NSM respectively, are equal. Thus, in order for the frictional force between the sleeve and the mandrel to be greater than the frictional force between the sleeve and the well bore the coefficient of friction of the sleeve with respect to the mandrel .mu..sub.SM, must be greater than the coefficient of friction of the sleeve with respect to the well bore .mu..sub.SW.
The coefficient of friction for contact between dry steel and rubber is somewhat higher than that between rubber and the wall of a fluid filling well bore; however, the coefficient of friction for contact between wet steel and rubber may be smaller. Thus, when there is water between the mandrel and the sleeve, the combined effects of lubrication by reducing the coefficient of friction and pressure balancing produce a frictional force between the sleeve and the mandrel that may be much less than that between the sleeve and well bore. In such instances, frictional forces applied to the sleeve by the well bore cause the sleeve to move with respect to the mandrel. Such movement can cause thickening of the sleeve at the upper end of the inflatable packer and can deform outwardly the upper reinforcing material. In some instances, the movement of the sleeve along the mandrel can cause the diameter of the packer to become greater than that of the borehole, in which case the packer becomes stuck.
A further failure mode due to the presence of fluid between the sleeve and mandrel has been noted and is believed to be due to the hydrodynamic forces acting on the packer as it is moved downwardly in the well bore. The movement of the packer through the well fluid within the well bore sets up regions of relatively higher and lower pressure axially along the length of the packer. Fluid between the sleeve and the mandrel flows from regions of higher pressure to those of lower pressure. These hydrodynamic forces tend to squeeze or milk fluid axially upwardly. In some instance, the movement of fluid between the sleeve and mandrel partially inflates the upper end of the sleeve to a diameter greater than that of the borehole, in which case, again, the packer becomes stuck.
It is therefore an object of the present invention to provide a method of preparing an inflatable packer for running in a fluid filled well bore which protects the packer from damage due to hydrostatic well bore pressures, but which prevents movement of the sleeve with respect to the mandrel.